The necessity for recovering waste heat energy from a variety of different furnace or engine types is becoming more acute. Prior art recovery systems have used principally two forms (a) a boiler system which absorbs heat units from hot exhaust gases, the heated water then being distributed for a variety of functional uses, and (b) pressurization of the heated exhaust gases so that the pressurized gases may drive mechanical equipment for extracting rotary mechanical energy. Both of these forms have proved to be expensive and have not obtained the degree of efficiency desired.
With respect to pressurization, one specific mode has utilized a Brayton cycle. A Brayton cycle is one which is comprised ideally of two adiabatic and two isobaric cycles, each alternated in sequence. Brayton cycle energy recovery systems have been utilized in industrial plants having furnaces giving off large volumes of hot gases, such as a steam system for a utility company. The known Brayton cycle system employs a compressor-heat exchanger-turbine combination. The compressor is initially driven by an exterior power source and later by the turbine; ambient air is introduced to the compressor, elevated in pressure, then transmitted through a heat exchanger which has been supplied with heat units from the furnace hot gases. The compressed heated air is then used to drive a turbine which in turn drives an electrical generator and the compressor. The spent compressed air is then mixed with ambient air for purposes of providing a preheated air supply to the furnace. The furnace exhaust gases, having passed through the heat exchanger, are then passed through a filtering system and returned to ambient air.
Several problems are associated with a direct Brayton cycle system including: (a) in the event a component of the compressor-heat exchanger-turbine apparatus fails, the furnace system is without an adequate supply of air irrespective of whether the air is heated or not heated and thus the furnace cannot be operated, (b) there is insufficient waste energy recovery principally limited by the maximum temperature of the materials employed to construct the heat exchanger, (c) the direct system experiences considerable leakage and significant exhaust gas back pressure contributing to low efficiency and may encourage toxic gases to enter the surrounding plant, (d) although the Direct Brayton Cycle has never been used for Cupola energy recovery, if it were to be so used, the high temperature of exhaust gases carried to the system would require the use of costly cooling equipment to reduce the temperature of the exhaust gases to a workable temperature for the heat exchanger (i.e. 1300.degree. F.), (e) the outlet temperature of the exhaust gases after having passed through the direct system is high contributing to a destruction a filtering system downstream thereof, (f) the direct system requires an unduly large heat exchanger to accommodate large furnace applications, and (g) the direct system is difficult to service.